In recent years, for the use in optical communication systems, optical modulators that use a semiconductor such as indium-phosphorus (InP) (hereinafter, “semiconductor optical modulators”) have been developed in place of optical modulators that use lithium niobate (LiNbO3) (hereinafter, “LN optical modulators”). Because it is possible to arrange electric field application efficiency of semiconductor modulators to be larger than that of LN modulators, it is easier to lower the driving voltage and to realize a compact design, with semiconductor modulators.
However, because semiconductors have a higher degree of optical confinement than lithium niobate (LiNbO3), the waveguide mode profile of semiconductors is smaller than that of lithium niobate. For this reason, when modulated light that is output from a semiconductor optical modulator is emitted from a waveguide into a space, the divergence angle of the modulated light is larger than the divergence angle of modulated light output from an LN optical modulator. In this situation, the modulated light output from the semiconductor optical modulator contains signal light and monitor light that monitors the signal light. The increase in the divergence angle of the modulated light is not desirable, because when the divergence angle is large, there is a possibility that interference may be caused between signal light beams and/or between monitor light beams.
To cope with this situation, a known optical transmitting apparatus is configured so that, for the purpose of preventing the divergence angle from becoming large, a plurality of collimate lenses are provided on the subsequent stage side of a plurality of waveguides that guide signal light and monitor light. In the optical transmitting apparatus, the plurality of collimate lenses are held by a holding member in such a manner that the optical axes of the plurality of waveguides are aligned with the optical axes of the plurality of collimate lenses, respectively. Further, the signal light and the monitor light that are emitted from the waveguides into the space are collimated by the plurality of collimate lenses. The signal light and the monitor light collimated by the plurality of collimate lenses are emitted from the plurality of collimate lenses in mutually the same emission direction.
Patent Document 1: Japanese Laid-open Patent Publication No. 2003-69504
According to the conventional technique described above, however, a problem arises where the apparatus becomes large-sized due to the positional relationship among the component parts along the emission direction of the signal light or the monitor light.
Let us further discuss this issue. According to the conventional technique described above, because the signal light and the monitor light are emitted from the plurality of collimate lenses in mutually the same direction, the positional arrangement of the component parts provided on the subsequent stage side of the plurality of collimate lenses is limited. For example, let us discuss a situation in which a polarization beam combining element configured to perform a polarization beam combining process on the signal light and a light receiving element configured to receive the monitor light are provided on the subsequent stage side of the plurality of collimate lenses. In that situation, the positional relationship between the polarization beam combining element and the light receiving element is limited by the emission direction of the signal light and the monitor light from the plurality of collimate lenses. As a result, there is a possibility that the apparatus may need to be large-sized in accordance with the positional relationship between the polarization beam combining element and the light receiving element along the emission direction of the signal light or the monitor light.